1. Field of the Invention
The present invention relates to an impedance-matching method and an impedance-matching circuit and more particularly, to an impedance-matching method and an impedance-matching circuit capable of matching the impedance between two circuits at different frequencies, which are suitably used for wireless communication systems using radio frequency (RF) signals.
2. Description of the Prior Art
Conventionally, impedance matching circuits have been used to maximize the performance of electronic devices used in RF circuits of wireless communication systems. FIG. 1 is a schematic circuit diagram showing an application example of conventional impedance-matching circuits.
In FIG. 1, first and second impedance-matching circuits 110 and 120 are connected to input and output terminals 132 and 133 of a RF circuit 140, respectively. The first impedance-matching circuit 110 serves to match or accord the output impedance of a RD circuit (not shown) located at a prior stage to the circuit 140 with the input impedance of the RF circuit 140. The second impedance-matching circuit 120 serves to match the output impedance of the RF circuit 140 with the input impedance of an RF circuit (not shown) located at a next stage to the circuit 140.
For the sake of simplification, the RF circuit 140 is illustrated as a RF amplifier equipped with only one npn-type bipolar transistor Tr in FIG. 1, which is an alternate-current (ac) equivalent circuit. The transistor Tr has an emitter connected to the ground, a base connected to the input terminal 132, and a collector connected to the output terminal 133.
The first impedance-matching circuit 110 is comprised of two coils or inductors 111 and 112. Two terminals of the inductor 111 are connected to the terminals 131 and 132, respectively. Two terminals of the inductor 112 are connected to the terminal 131 and the ground, respectively. The circuit 110 has a so-called "L--L matching" configuration. The second impedance-matching circuit 120 is comprised of two capacitors 121 and 122. Two terminals of the capacitor 121 are connected to the terminals 133 and 134, respectively. Two terminals of the capacitor 122 are connected to the terminal 134 and the ground, respectively. The circuit 120 has a so-called "C--C matching" configuration.
The first impedance-matching circuit 110, which serves to match the output impedance of the prior-stage RF circuit with the input impedance of the RF circuit 140, has a problem that impedance matching is realized only at single frequency. Thus, to realize impedance matching at different frequencies between the RF circuit 140 and its preceding-stage circuit, some contrivance is needed. This is applied to the second impedance-matching circuit 120 also.
An example of the contrivance is explained below with reference to FIG 2, which shows a schematic circuit configuration of a single superheterodyne receiver of a portable phone of the Personal Digital Cellular (PDC) type that has been used in Japan.
In the receiver circuit of FIG. 2, an antenna 101 receives a RF signal in the 820 MHz band of frequencies. A RF amplifier 102 amplifies the RF signal received by the antenna 101 to produce an amplified RF signal. A frequency mixer 103 frequency-mixes the amplified RF signal from the RF amplifier 102 with a local signal of 950 MHz sent from a local oscillator 104, producing an intermediate Frequency (IF) signal of an IF frequency 130 MHz which is equal to the difference between the two frequencies of 950 MHz and 820 MHz. An IF amplifier 105 amplifies the IF signal from the frequency mixer 103 to produce an amplified IF signal. A demodulator 106 demodulates the amplified IF signal from the IF amplifier 105 according to the specified demodulation method, thereby deriving the transmitted information from the amplified IF signal.
In the circuit of FIG. 2, if two adjacent ones of the RF circuits handling the RF signal, such as the RF amplifier 102, the frequency mixer 103, the local oscillator 104, and the IF amplifier 105, are connected to each other through the conventional impedance matching circuit 110 or 120 shown in FIG. 1, the configuration of the impedance matching circuit 110 or 120 is designed in such a way that the impedances of the two RF circuits to be connected are matched with each other at a specific single frequency (e.g., 820 MHz) within the frequency band (i.e., the 820 MHz band) of the received signal. In this case, the RF amplifier 102 has a frequency response or characteristic of the Voltage Standing-Wave Ratio (VSWR) shown in FIG. 5, where f is the frequency of the received signal. In other words, since the necessary band of frequencies is only the 820 MHz band, the configuration of the impedance matching circuit 110 or 120 is designed so that the output impedance of one of the RF circuits to be connected is equal to the input impedance of the other at a frequency of 180 MHz.
In recent years, however, technological advances have been rapidly accomplished in the radio communication equipment and systems and as a result, there has been the need to enable RF receivers to handle the RF signals within two separated bands of frequencies. An example of the RF receivers coping with this need is a telephone capable of handling the RF signal in the 820 MHz band used for the PDC-type portable phone system and that of the 1.9 GHz (i.e., 1900 MHz) band used for the Personal Handy-phone System (PHS). Two examples of the conventional circuit configurations of this two-band telephone is shown in FIGS. 3 and 4.
In the circuit configuration of FIG. 3, there are provided with a circuit block for handling the received signal in the 820 MHz band comprising a RF amplifier 102a, a frequency mixer 103a, a local oscillator 104a, and an IF amplifier 105a, and a circuit block for handling the received signal in the 1900 MHz band comprising a RF amplifier 102b, a frequency mixer 103b, a local oscillator 104b, and an IF amplifier 105b. The two circuit blocks for the 820 MHz and 1900 MHz bands are alternatively used by switches 107 and 108. The local oscillators 104a and 104b generate local signals having local frequencies of 950 and 1770 MHz, respectively.
In the circuit configuration of FIG. 3, each of the RF amplifiers 102a and 102b provides the VSWR-f characteristic shown in FIG. 6A. Specifically, impedance matching is carried out only at a specific frequency (e.g., 820 MHz) within the 820 MHz band with respect to the circuit block for the 820 MHz band. Simultaneously with this, impedance matching is carried out only at a specific frequency (e.g., 1900 MHz) within the 1900 MHz band with respect to the circuit block for the 1900 MHz band.
Since the two circuit blocks for the 820 MHz and the 1900 MHz bands are alternatively used by the switches 107 and 108 according to the frequency band of the received signal, the VSWR-f characteristic of each of the RF amplifiers 102a and 102b is given by the curve shown in FIG. 6B produced by combining the two curves in FIG. 6A with each other.
In the circuit configuration of FIG. 4, which is a variation of the configuration of FIG. 3, there are provided with a common frequency mixer 103 and a common IF amplifier 105 for handling the received signals in the 820 and 1900 MHz bands instead of the dedicated local oscillators 104a and 104b and the dedicated frequency mixers 105a and 105b in FIG. 3 Also, according to this difference, a switch 109 for selecting one of the outputs of the RF amplifiers 102a and 102b and a switch 110 for selecting one of the outputs of the local oscillators 104a and 104b provided instead of the switch 108 in FIG. 3. The other part of the circuit configuration of FIG. 4 is the same as that of FIG. 3. In this case, similar to the configuration of FIG. 3, each of the RF amplifiers 102a and 102b provides the VSWR-f characteristic shown in FIG. 6B.
With the circuit configurations shown in FIGS. 3 and 4, however, switching means (i.e., the switches 107, 108, 109, and 110) are necessarily provided. Thus, there is a problem that possible electric-power loss of the telephone due to the switching means is larger than the circuit configuration of FIG. 2 designed for a single band of frequencies. Moreover, two dedicated circuit blocks need to be provided for the 820 and 1900 MHz bands and therefore, there is another problem that the circuit configuration of the telephone is more complicated than that of FIG. 2.
Accordingly, to solve the above-described problems in the circuit configurations of FIGS. 3 and 4, a design to lower the Q value of the impedance matching circuit 110 or 120 thereby realizing approximate impedance-matching within a frequency range covering both the 820 and 1900 MHz bands may be used in the circuit configuration of FIG. 2. In this case, the VSWR-f characteristic of the RF amplifier 102 is given by the curve shown in FIG. 7. Although the VSWR-f characteristic of FIG. 7 is unable to provide complete impedance-matching (i.e., matching at optimum impedance values) in the whole frequency range covering both the 820 and 1900 MHz bands, it enables the single-band circuit of FIG. 2 to realize approximate impedance-matching within the same frequency range.
However, the design to utilize the VSWR-f characteristic of FIG. 7 has the following problems.
A first one of the problems is that the impedance is unable to be set at an optimum value for each of the 820 and 1900 MHz bands, which is due to the following reason. Specifically, in general, the impedance matching circuit 110 or 120 is configured so that the impedance is completely matched at a medium frequency of 1360 MHz between the values of 820 MHz and 1900 MHz, thereby equalizing the impedance-matching level in the 820 and 1900 MHz bands.
A second one of the problems is that the signal-receiving performance cannot be optimized compared with single-band configuration using the VSWR-f characteristic of FIG. 5, resulting in increase in possible electric-power loss of the receiver circuit. This problem is caused by the intentionally-decreased Q value of the impedance matching circuit 110 or 120 and by the increased electric-power loss of the circuit 110 or 120 due to decrease of the Q value.
As described above, with the conventional impedance matching circuit 110 or 120 shown in FIG. 1, it is obvious that the impedance of each of two RF circuits to be connected is unable to be set at optimum values at two separate frequencies. Also, the electric-power loss is increased due to impedance matching.